In X-ray imaging, a film/screen system (F/S) having a film and intensifying screen inserted in a cassette has been conventionally used as an X-ray sensor which acquires an X-ray image of a subject to be examined.
Recently, an X-ray sensor has been proposed, which can directly convert an X-ray image into a digital output in real time. For example, there is available an X-ray detector formed by stacking a solid-state photodetector and a phosphor on a substrate made of quartz glass with an amorphous semiconductor being sandwiched between them. The solid-state photodetector is formed by arranging solid-state photodetecting elements, each comprised of a transparent conductive film and conductive film, in the form of a matrix. The phosphor converts X-rays into visible light. In the acquisition process for a digital X-ray image using this X-ray detector, when the X-ray detector is irradiated with X-rays transmitted through a target, the X-rays are converted into visible light by the phosphor, and the visible light is detected as an electrical signal by the photoelectric conversion portion of each solid-state photodetecting element. The electrical signal obtained in this manner is read out from each solid-state photodetecting element by a predetermined read method, and is A/D-converted, thereby obtaining an X-ray image signal. The above X-ray detector is disclosed in detail in Japanese Patent Laid-Open No. 8-116044. Many X-ray detectors have also been proposed, which are designed to directly acquire X-rays through a solid-state photodetector without using any phosphor. In addition, there have been proposed many X-ray detectors designed to acquire a digital X-ray image by irradiating a storage phosphor, which is a special phosphor, with an X-ray signal, causing optically stimulated luminescence using a laser, and detecting optically stimulated luminescence light through each photodetecting element.
Phosphor plates using phosphors which are used in X-ray detectors include a phosphor plate formed by using a powder phosphor and a phosphor plate formed by crystal-growing a phosphor into a needle shape. In general, as a phosphor increases in thickness (mass thickness or the like), the X-ray absorption efficiency increases, but the resolution deteriorates. If a phosphor is crystallized into a needle shape, light emitted inside the phosphor is transmitted to the solid-state detector through a needle-like crystal serving like an optical fiber. Even if therefore, the phosphor increases in thickness, the resolution deteriorates less. Therefore, a phosphor plate formed by crystal-growing a phosphor into a needle shape is characterized in that it has high X-ray absorption efficiency and high resolution.
When a phosphor plate is to be formed by coating with a powder phosphor, a large-area phosphor plate without performance unevenness can be formed at low cost. In contrast to this, when a phosphor plate is to be formed by crystal-growing a phosphor into a needle shape, the phosphor must be grown into a needle shape by vacuum evaporation. It is difficult in terms of manufacturing techniques to form a large-area phosphor plate without performance unevenness at low cost. The presence of performance unevenness in an X-ray detector will adversely affect image quality. One of such performance unevenness, in particular, is a resolution distribution in which different resolutions appear at different positions on the phosphor plate. This resolution distribution leads to different sharpnesses of image quality on the central portion and periphery, and hence is undesirable in terms of X-ray image diagnosis.
FIGS. 11A and 11B are views for explaining the resolution distribution of the above X-ray detector, and more specifically, graphs showing presampling MTFs indicating the resolutions of the X-ray detector. For presampling MTF (Modulation Transfer Function), see Med. Phys., 11(3), 287-295, 1984, and Hatagawa et al., “Study on MTF Measurement in Digital System Using Rectangular Wave Chart”, Japanese Journal of Radiological Technology, Vol. 53, No. 11.
FIG. 11A shows the resolutions near the center and end portion of the X-ray detector. The abscissa represents the spatial frequency (unit: lp/mm: the abscissa representing the number of pairs of white and black lines existing within 1 mm); and the ordinate, the presampling MTF. The solid curve represents the resolution near the center of the X-ray detector. The broken curve represents the resolution near an end portion of the X-ray detector. The resolution distribution of the X-ray detector shown in FIG. 11A indicates that the resolution at the center is superior to the resolution at the end portion in each spatial frequency band.
FIG. 11B shows the distribution of resolutions from near the center of the X-ray detector to near the end portion. The abscissa represents the distance from the center of the X-ray detector (if the X-ray detection surface of the X-ray detector is rectangular, the center indicates the intersection of diagonals of the rectangle); and the ordinate, the presampling MTF. Each data sequence indicates the values of presampling MTFs at spatial frequencies at intervals of 1.0 lp/mm. As is obvious from the graph of FIG. 11B, the presampling MTF gradually changes from near the center, and the presampling MTF near the center is superior to that at the end portion throughout all the spatial frequencies.
As is understood from the case shown in FIGS. 11A and 11B, the resolution of the X-ray detector changes from the center of the X-ray detector concentrically in accordance with changes in radius. When a phosphor Plate is to be formed by crystal growth, a phosphor must be crystal-grown by vacuum evaporation. The uniformity of a performance distribution greatly depends on the size of an evaporation furnace. As the size of an evaporation furnace increases, the price of a phosphor increases. In practice, therefore, it is difficult to form a large-area phosphor plate without performance unevenness.
On the other hand, doctors who diagnose X-ray images have seen many X-ray images, and hence are sensitive to changes in the image quality of X-ray images. Therefore, improvements must be done to changes in resolution like those shown in FIGS. 11A and 11B.